1. Field of the Invention
This invention relates to electronic circuitry, and more particularly to a trimming circuit suitable for use with an integrated circuit digital to analog converter (DAC).
2. Description of the Prior Art
Most integrated circuit devices such as DAC's are manufactured initially with a certain tolerance for error, and their operating characteristics will generally differ from the nominal specifications. After manufacture the devices are tested for accuracy, and their end use determined by how close their actual operating characteristics come to the desired specifications. Frequently it is necessary to discard the device entirely because of failure to come within the required tolerance range.
Due to the difficulty in matching component values, it is common to initially manufacture a high accuracy DAC to a somewhat less stringent tolerance, and then trim at least its more important stages to the desired accuracy. In this way the operating characteristics of the device as a whole are upgraded, and some devices which might otherwise have been discarded may be retained.
In one popular trim method a laser beam is employed to physically remove portions of the bit current determining resistors, and thereby modify the values of the bit currents.
Another technique employs a set of parallel connected resistors for each stage to be trimmed. One side of the resistors of each set are connected to a voltage source and the other side to the converter stage, such that a trimming current having a magnitude determined by the overall resistance of the parallel resistance network is added to the stage outputs. The resistance values of the individual resistors for each set are weighted such that a plurality of discrete trimming levels can be achieved, depending upon which resistors are included in the circuit. For this purpose each resistor is connected in series with a switch which allows it to be either included or removed from its associated trimming circuit. Various switching devices may be used, so long as they are capable of being controlled independently. One approach provides metal links in series with each resistor, and employs a laser to cut the links of those resistors which are to be removed from the trimming circuit.
Although the individual stage accuracies can be considerably enhanced with the use of resistive trimming circuits, this type of trimming is not ideally suited for use with integrated circuits. The resistors take up relatively large areas on the surface of the circuit chip, and are themselves subject to manufacturing inaccuracies which can introduce errors into the stage output even after trimming. Since it cannot be known before manufacture whether a particular stage will require positive or negative trimming, provision for both polarities has to be made. Also the laser equipment required to implement the above schemes is relatively large and expensive.
Accordingly, while presently available trimming arrangements do improve DAC accuracy and yield, there is still considerable room for improvement.